I'm interested in this as well. Honestly, though, I don't think there are a lot of options for decent baby consignment. I've done both Ebay and ThredUp and been pretty disappointed, even when sellling high-quality items.
For the baby clothes that came to me used, I've found that the path of least resistance is to give them a week on my city's "baby classifieds" facebook group. If I don't get any takers, I'll donate to our local YWCA, which runs a shelter and a thrift store.